The present invention relates to a floating offshore structure such as an offshore oil platform or barge, with an angle of oscillation i at most equal to a predetermined value io which is generally between about 5xc2x0 and 10xc2x0.
As is well known, cross-corrugated packing is used in certain distillation columns in the place of the distillation plates in order to provide for the exchange of material and heat between an ascending gas and a descending liquid. This cross-corrugated packing consists of a superposition of lengths. Each length is formed from a stack of corrugated strips each one arranged in a vertical general plane and one against the next. The corrugations are oblique and descend in opposite directions from one strip to the next.
The strips generally comprise dense small-diameter perforations, with a degree of perforation of about 10%, to allow the liquid to pass through the corrugated strips.
GB-A-1,004,046 and CA-A-1,095,827 describe such cross-corrugated packing.
Cross-corrugated packing is generally made from a flat product, namely from metal sheets in the form of strip. The strips are first of all folded (or bent) to form a corrugated sheet metal strip, the corrugations of which are oblique with respect to the axis of the strip. The folded strips are then cut into lengths and then stacked, turning every alternate strip around. The lengths of packing thus obtained are often called xe2x80x9cpacksxe2x80x9d.
WO-A-90/10497 describes, inter alia, a packing similar to the aforementioned cross-corrugated packing but perforated in a different way. The term xe2x80x9ccross-corrugated packingxe2x80x9d used here also includes such packing, together with any similar packing.
Offshore oil platforms produce residual gases. For economic and environmental reasons, it is becoming increasingly necessary to make profitable use of these gases. One method consists in converting them into heavier hydrocarbons, in the form of a liquid which is therefore more readily transportable, using the Fischer-Tropsch process, which consumes vast amounts of oxygen.
It would therefore be desirable to be able to install a distillation column with cross-corrugated packing on such a floating structure, but serious difficulties are encountered in so doing.
This is because, in cross-corrugated packing, the successive packs are rotated through a certain angle, generally 90xc2x0, about the axis of the column from one pack to the next, so as to encourage periodic redistribution of the liquid over the entire cross section of the column.
In consequence, at every moment, the corrugated strips of certain packs at least form an angle of less than 45xc2x0 with the plane of inclination. For these strips, the inclination i therefore has a substantial component in their general plane, and tends to cause the descending liquid to be deviated. Liquid thus tends to reach the outer shell of the column, and then to trickle down along this shell without playing a part in the distillation. The distillation performance of the column is consequently degraded.
The basic problem that the invention sets out to solve consists in producing a floating marine structure bearing a distillation column with cross-corrugated packing capable of operating satisfactorily in the presence of oscillations due to the heave and the amplitude of which oscillations is typically between 5xc2x0 and 10xc2x0, possibly with a predominant direction of oscillation which is due to the prevailing winds. An essential feature is thus that the liquid distributed at the top of the column should more or less uniformly wet the packing over the entire cross section of the column despite the aforementioned oscillations.
To this end, the subject of the invention is an floating offshore structure such as an offshore oil platform or barge, with an angle of oscillation i at most equal to a predetermined value io, which is generally between about 5xc2x0 and 10xc2x0, characterized in that it bears a distillation column equipped, over at least part of its length, with a cross-corrugated packing, this packing comprising a superposition of lengths of packing, each of which comprises a stack of vertical corrugated strips with their corrugations alternately inclined in opposite directions, the configuration of the corrugation of the strips of at least one length of packing being chosen to be such that, for each pair of adjacent strips of this length, when the strips are inclined in their general plane by the said angle i, the mean lines of greatest slope of the mean corrugation flanks of the two strips delimit, measuring from any point on the upper edge of the pair of strips, on each side of the perpendicular to this edge at the said point, a flattened cone, the axis of which forms with the said perpendicular an angle d such that the ratio d/i is less than 0.6 and preferably less than 0.5.
The floating off shore structure thus defined may have one or more of the following characteristics, taken in isolation or in any technically feasible combination:
the mean angular aperture xcex3 of the corrugation and the inclination xcex4 of the generatrices of the corrugations of each of the strips of the said length, when viewed face on, are connected by the following relationship:             1      -              2        ⁢        sin        ⁢                  xe2x80x83                ⁢        2        ⁢        δ        ⁢                  xe2x80x83                ⁢                  cos          2                ⁢                  γ          2                ⁢                              B            -                                          sin                2                            ⁢                              γ                2                                                          2            ⁢            B            ⁢                                          B                -                                  A                  2                                                                           less than     0.6    ,                    with:              ⁢      A        =          1      -                        cos          2                ⁢                  δ          ·                      cos            2                          ⁢                              y            _                    2                                        and:              ⁢      B        =                  A        ⁡                  (                      1            +                                          sin                2                            ⁢                              γ                2                                              )                    -                        sin          2                ⁢                  γ          2                    
xcex3=40xc2x0 approximately and xcex4 is between about 30xc2x0 and about 60xc2x0, preferably between about 40xc2x0 and about 60xc2x0;
xcex3=50xc2x0 approximately and xcex4 is between about 40xc2x0 and about 60xc2x0, preferably between about 50xc2x0 and about 60xc2x0;
xcex3=60xc2x0 approximately and xcex4 is between about 40xc2x0 and about 60xc2x0, preferably between about 50xc2x0 and about 60xc2x0;
xcex3=70xc2x0 approximately and xcex4 is between about 50xc2x0 and about 60xc2x0 and preferably close to 60xc2x0.
xcex3=80xc2x0 approximately and xcex4=60xc2x0 approximately;
the column is a double air distillation column.